FORMATION OF 1,2,4,5,6,8-HEXAMETHYLANTHRACENE AND ITS RADICAL-CATION FROM 2,2',3,4',5,5',6-HEPTAMETHYLDIPHENYLMETHANE IN TRIFLUOROACETIC-ACID

Upon dissolution in trifluoroacetic acid, 1,2,4,5,6,8-hexamethylanthracene (HM A) gives an orange red solution due to the presence of its protonated form. Under the influence of diffuse daylight, the radical cation of HMA was slowly formed and detected by its EPR spectrum. The hyperfine splitting constants were determined to be 0.543 (9,10-H), 0.079 (3,7-H), 0.348 (1.4,5,8-Me4) and 0.231 (2,6-Me2) by combining the EPR and ENDOR methods and the assignments were confirmed by the shapes of the EPR spectra of HMA.+ in TFA/dichloromethane (line-broadening) and TFA-d (exchange of the four 3,7,9, 10-hydrogens). The same EPR spectrum was obtained by keeping 2,2',3,4',5,5',6-heptamethyldiphenylmethane in TFA solution for an extended period. The ENDOR spectrum of HMA.+ in TFA was strongly temperature dependent, in that eight additional hf splittings due to a second radical appeared at lower temperatures (255-237 K). This fully reversible phenomenon is ascribed to equilibrium (i) being established at low temperature, the point of attack by trifluoroacetate ion being the 9-position.